complain about Reviewed-by footers

unlike previously thought, gerrit won't eliminate known bogus footers,
so they would stay behind. consequently, complain about them loudly.

note that the footers are just fine above a "(cherry picked from ...)"
line, as we want to preserve the original approvals.

Change-Id: I7856810adb3e3b3e91e539eb626375dd32648460
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>